1. Field of the Invention
The invention relates to a device for the substance-specific treatment of a fluid having
a) a casing, PA1 b) an inlet arrangement for feeding the fluid to be treated into the casing, PA1 c) an outlet arrangement to remove the treated fluid from the casing and PA1 d) at least one treatment element for the substance-specific treatment of the fluid with one end facing the inlet arrangement and the other end facing the outlet arrangement, PA1 and the use of this device. PA1 a) feeding the fluid to be treated into the casing, PA1 b) causing the same fluid to flow through the casing, whereby the fluid to be treated is caused to flow along the exterior of the membrane as a primary stream but not along its interior, in such a fashion that a part of this primary stream flows as a secondary stream into the membrane via the exterior, through the membrane, where the substance-specific treatment of the fluid takes place on the part of the fluid to be treated which forms the secondary stream, and then flows out through the interior of the membrane, and PA1 c) removal of the treated fluid from the casing. PA1 a casing (25); PA1 an inlet arrangement (24) for introducing the fluid to be treated (23) into the casing (25); PA1 an outlet arrangement (28) to remove the treated fluid (27) from the casing (25); and PA1 at least one treatment element for the substance-specific treatment of the fluid with one end of the at least one treatment element facing the inlet arrangement and the other end facing the outlet arrangement, PA1 a) feeding the fluid to be treated into the casing, PA1 b) causing the same fluid to flow through the casing, PA1 c) removing the treated fluid from the casing,
The invention also relates to a process for the substance-specific treatment of a fluid using a semipermeable membrane with a porous structure arranged in a casing, where the membrane has at least a first surface defining its exterior and at least a second surface defining its interior, comprising at least the steps:
2. Discussion of Related Art
Substance-specific treatments of fluids are gaining an ever higher degree of importance in application fields such as biotechnology, medicine or chemical engineering. Examples of this are the recovery of active agents from cell suspensions in which genetically modified cells have produced substances such as antibodies, hormones, growth factors or enzymes, usually in small concentrations. An important application is also the extracorporeal removal of undesired substances from human blood. Finally, a broad field of application is the catalytic or biocatalytic--enzymatic--treatment of liquids such as the hydrolysis of oils by lipases which are immobilized on a matrix. In many applications for the treatment of liquids, these contain particles of many different kinds, i.e. they are suspensions.
The substance-specific treatment of fluids is often conducted such that the fluid to be treated is brought into contact with a carrier material on and/or in which interacting groups or substances are immobilized, which interact in specific, selective fashion with the target substance contained in the fluid, i.e., the substance targeted by the substance-specific treatment. Interactions of this kind may for example be cation or anion exchange, hydrophilic-hydrophobic interactions, hydrogen bonding, affinity or enzymatic or catalytic reactions, etc. In affinic substance separation, ligands are coupled to the carrier material or immobilized in the carrier material whose function it is to specifically bind a single target substance or a whole class of substances by adsorption. This target substance is termed a ligate. An example of a class-specific ligand are positively charged diethylaminoethyl (DEAE) groups or negatively charged sulfonic acid (SO.sub.3) groups, which adsorb the class of positively charged or negatively charged molecules, respectively. One example of specific ligands are antibodies against a certain protein which is bound to the antibody as a ligate.
The basic criteria in the substance-specific treatment of fluids are productivity and selectivity. With regard to productivity, it is important that as many groups with a substance-specific action as possible which can interact with the target substance in the fluid to be treated are available per unit volume. A simultaneous aim is the maximization of the transport of the target substance to the groups or substances with a substance-specific action.
A carrier material for ligands which is frequently employed in affinity chromatography are sepharose particles to which the ligands are coupled and which are arranged in the form of a packing in a chromatographic column. Although it is possible by this means to achieve a high concentration of ligands with high selectivity, it is well-known that the productivity is low since due to the compressibility of the sepharose particles the flow through the column must remain relatively low. Moreover, the access of the ligates to the ligands contained in the sepharose particles is controlled by diffusion, so that especially for the separation of larger molecules such as proteins, due to their low diffusion rates the residence times are long, resulting in low throughput rates and low productivity.
U.S. Pat. No. 4,202,775 discloses a column material consisting of porous rigid polymer particles which can be used as adsorbents in the separation of organic components which are adsorbed on proteins present in aqueous solution. Although this column material no longer displays the disadvantage of compressibility, there remains the disadvantage of the diffusion-controlled substance transport in the particles, coupled with long residence times and low productivity.
In U.S. Pat. No. 5,019,270, a chromatographic column material made from rigid porous particles is presented in which a partial stream of the fluid to be treated and flowing through the chromatographic column flows through these particles by convection, whereby it comes into contact with the interacting groups in the porous structure of the particles and is then reunited with the liquid stream flowing around the particles. Due to the convective manner of the substance transport through the particles a reduction in the residence time and an increase in productivity is possible in comparison to the column material described earlier.
Although it is an advantage of chromatographic columns filled with particles of this kind that their structure and use is very simple, all particle-shaped column materials have the disadvantage in common that when treating liquids containing particles or cells, i.e. suspensions, the size of the particles of the column material selected must be relatively large in order to guarantee good flux through the particle packing without keeping back particles or, for instance, cells of the liquid, without any deep filtration effect through the packing, and in order to keep the pressure loss as low as possible. However, this means that the quantity of groups with a substance-specific action available per unit volume is reduced since the volume taken up by the particles in the column is smaller. In addition, increased particle size is associated with a detrimental prolongation of the residence time.
In chromatographic columns of this kind the goal is for the particles to be present in an ordered packing in the column, thus maximizing the proportion of particles in the packing and causing the flux between the particles to be more uniform. This can be partly achieved by using spherical particles with as uniform a diameter as possible. However, the manufacture of such uniform particles is laborious. In addition, when such particle packings are used it is generally not possible to prevent the shape of the flow channels between the particles from being non-optimal in fluid dynamic terms. In the gaps between the particles, it is easily possible for dead spaces to arise which can lead to negative phenomena particularly in the substance-specific treatment of suspensions such as the deposition of suspended particles in the gaps between the particles of the packing.
The described disadvantages of the particle-shaped carrier materials gave rise to the development of a series of processes for the substance-specific treatment of fluids in which membranes with a porous structure are employed as carrier materials for the interacting groups. Due to their porous structure, these membranes provide a large interior surface so that a large number of functional groups can be coupled onto the membranes in a high concentration per unit volume, which interact with the fluids to be treated flowing through the membrane. See, for example, E. Klein, "Affinity Membranes", John Wiley & Sons, Inc., 1991; S. Brandt et al, "Membrane-Based Affinity Technology for Commercial Scale Purifications", Bio/Technology Vol. 6. (1988) pp. 779-782.
By means of the design of the membrane used, it is possible to adapt it to the requirements of the treatment process. There are membranes in the form of hollow-fibers or flat membranes made from a wide variety of materials, so that it is possible to achieve an adaptation to the physicochemical properties of the fluids to be treated. The pore size of the membranes can also be adjusted such that the fluid to be treated with the target substance it contains can flow through the membrane by convection and no blocking of the membrane occurs in case the target substance is bound to the interacting groups.
By means of the thickness of the membrane wall, the residence time of the fluid to be treated can be influenced, as can the pressure loss which arises as it flows through. Here, membranes are characterized by short transport distances for the fluid to be treated to the interacting groups, due to their usually small wall thickness (for example, &lt;100 .mu.m), which makes the residence times relatively short. At the same time a further advantage associated with the use of membranes as carrier materials as opposed to those in the form of particles is the more uniform flux through the carrier material due to the essentially uniform thickness of the membrane wall, and consequently a more narrow distribution of residence times as well as more uniform and more complete "utilization" of the interacting groups results.
A series of devices containing such membranes is described which are used in processes for the substance-specific treatment of fluids and in which both flat membranes and hollow-fiber membranes are employed. Here, so-called dead-end filtration or dead-end modules must be distinguished from cross-flow filtration or cross-flow modules.
In dead-end filtration, the entire fluid flowing into the membrane module as the feed stream is made to pass through the membrane and led off as a filtrate or permeate at the downstream side of the membrane opposite the upstream side.
In cross-flow filtration, the feed stream flows parallel to one side of the membrane, whereby part of the feed stream enters and flows through the membrane. The partial stream flowing through is led off as the permeate, and the partial stream remaining on the feed stream side is led off as the retentate. Here, an additional fluid stream can also be introduced on the permeate side of the membrane which takes up the partial stream flowing through the membrane.
In U.S. Pat. No. 4,935,142, a device for conducting an affinity separation process in dead-end fashion is described which contains piles of flat membranes. Coupled to the flat membranes are ligands to which the ligates to be separated from the fluid to be treated are bound. The flat membranes forming the membrane pile are sealed against the casing around them so that the stream is forced to flow through the membrane pile. The disadvantage of a construction of this kind is the high pressure loss on flowing through the pile, making additional measures also necessary to give the flat membrane elements adequate stability to withstand the high pressures which develop.
Also, in EP-A-0 173 500, EP-A-0 280 840 and EP-A-0 610 755, devices for use in membrane-based affinity separation processes, such as the isolation of immunoglobulins, antigens, etc., are described. These devices or membrane modules contain microporous flat membranes folded in the form of a star. The flat membranes folded in the form of a star are supported between two coarse meshes and positioned between two cylindrical casing elements which are arranged coaxially to each other. Preferably, several flat membranes folded in the form of a star are arranged concentrically to each other in the casing. Modules constructed in similar fashion are described in EP-A 0 662 340, whereby small particles with specifically interacting groups are incorporated into the folded flat membrane structure.
In the devices mentioned, the liquid to be treated is made to pass through the module from the inside to the outside or in the opposite direction under the force of pressure and at the same time in dead-end fashion flows through the membrane convectively. In contrast to modules with unfolded concentrically arranged membranes, the modules mentioned have the advantage that the membrane surface is larger while at the same time the pressure loss is lower. However, usually only low filling levels, defined as the membrane volume relative to the total volume of the module, are possible.
The disadvantage associated with all membrane modules run in dead-end fashion is that they are not suitable for the treatment of fluids containing particles, i.e., suspensions for example, if the particles contained in the fluid are of the magnitude of the pore diameter. The particles would cause a layer to form on the membrane wall and block the membrane. For use in affinity separation processes, for example, for liquids containing particles, i.e., suspensions, such dead-end modules can only be run in combination with a pre-filtration/pre-purification stage arranged in series in front. However, this means that the efficiency of a process of this kind is reduced, for instance due to the fact that a large part of the target substance is often lost in the pre-purification.
The disadvantages of modules run in dead-end fashion mentioned with regard to, for example, their use for suspensions can at least partly be avoided by using cross-flow modules. In these, the formation of a layer of suspended particles can be reduced by means of the feed stream flowing parallel to the membrane surface if the shear stresses are high enough.
WO 90/05018 discloses a membrane module for use in affinity separation processes whose construction corresponds to a cross-flow module. A liquid containing ligates is introduced into the module casing via an inlet arrangement and flows tangentially over one side of a membrane, which may for example be one made from hollow-fibers, to which ligands are coupled. Part of the liquid enters the membrane, flows through it, whereby the ligates are added to the ligands, and leaves as a permeate stream by the membrane side opposite the side it entered. Via separate outlet arrangements, the retentate stream and the permeate stream are led off. An essential characteristic of the membranes used in accordance with WO 90/05018 is an isotropic, microporous structure which makes a convection stream of solutions containing macromolecules possible.
In U.S. Pat. No. 4,266,026, a cross-flow module containing anisotropic hollow-fiber membranes is described for a process for conducting catalytic reactions. The catalysts used here are primarily enzymes which are immobilized in the membrane structure via coupling reagents, for example. The liquid to be treated flows as the feed stream under pressure through the lumen of the hollow fibers. Part of the liquid thereby flows convectively through the membrane wall and is subjected to the catalytic reaction. An example given is the catalytic conversion of lactose into glucose and galactose by means of galactosidase as a catalyst. The retentate and the permeate are led off from the module as separate streams of liquid, reunited in a storage container and recycled to the module on the lumen side until the desired turnover of the reacting substance has been reached.
A variation of a cross-flow process is described in WO 93/02777. For the specific removal of certain components from blood, a U-shaped bundle of semipermeable hollow-fiber membranes embedded in a specially shaped casing is employed, which acts as a plasma filter. The blood flows through the hollow-fiber membranes on the lumen side, and the blood plasma separated off by means of the membrane undergoes the substance-specific treatment in the exterior space around the hollow-fiber membranes. In this exterior space, there is a cleansing medium containing, for example, immobilized enzymes or antibodies for depositing the components to be separated. In principle the bundle can be divided up into an inlet arm and an outlet arm. Due to the positive transmembrane pressure arising in the area of the inlet arm, a convective transport of blood plasma (which superimposes diffusion) takes place through the membrane into the exterior space. In the area of the outlet arm, the treated plasma flows back into the lumen of the hollow-fiber membranes due to the negative transmembrane pressure arising there, and is reunited with the blood.
The advantage offered by the process according to WO 93/02777 is the fact that no separate pumps and/or regulatory organs are required for the permeate stream, i.e., the plasma stream. However, the modules used have a large volume of dead space in the exterior space of the membranes.
In the device for treating blood according to EP-A-0 112 094, the permeate is not removed separately from the module used either, but is reunited with the retentate, i.e. here, the blood, inside the module. Here, too, the membrane is being used as a plasma separator, a channel being formed from one side of the membrane through which the blood flows, and from the other side of the membrane and the casing surrounding it a treatment space is formed containing a material for the substance-specific treatment. By means of a suitable device, pressure variations are applied to the treatment space. This periodically changes the pressure difference between the blood channel and the treatment space such that, in alternate fashion, plasma firstly permeates from the blood through the membrane into the treatment space where it is subjected to the substance-specific treatment, and then the treated plasma flows in the opposite direction back through the membrane and is reunited with the blood.
A similar principle is pursued in WO 80/02805. According to this publication, too, part of the feed stream is made to pass by convection through the membrane and back again by means of pressure oscillations. In contrast to EP-A-0 112 094, in the device according to WO 80/02805, the biologically active material to which the target substance of the liquid to be treated is to be bound is coupled to the membrane in the pores and at the surface of the membrane, so that the substance-specific treatment of the liquid takes place while it is flowing through the membrane.
In EP-A-0 341 413, an adsorber module for the treatment of whole blood is described, in which blood flows in cross-flow fashion on the lumen side through the hollow-fiber membranes contained in the module which are equipped with ligands. Here, the plasma goes as a permeate through the hollow-fiber membrane wall into the exterior space around the hollow-fiber membranes, whereby the plasma is treated within the membrane wall. In a special embodiment, this module does not have any outlet for the permeate, but rather the plasma separated as the permeate is collected during the whole blood treatment in the exterior space around the capillaries and due to the pressure situations which arise passes through the hollow-fiber membrane wall again into the lumen of the hollow-fiber membrane. The disadvantage of a module design of this type, however, is that the plasma stream passing through the membrane can only to a limited extent be influenced. In addition, the necessary treatment times are relatively long, since filling up the exterior space around the hollow-fiber membrane itself requires permeate times of more than 10 minutes.
The modules described for the substance-specific treatment of liquids being run in cross-flow fashion have disadvantages such as the necessity for additional pumps and/or regulatory organs due to the separate permeate and retentate streams or additional aggregates for producing the pressure oscillations. They all have the disadvantage in common that the membranes must always be embedded in the casing, making manufacture of the membrane modules laborious. The effect of this is particularly negative if the substance-specific treatment makes it necessary to connect several modules in series. This is also important if for coupling or polymerizing of the ligands aggressive solvents such as toluene are used and this coupling takes place in the device. In practical use it has emerged that an embedding method providing the necessary resistance to the solvent is very laborious and expensive to achieve.